1. Field of the Invention
The present invention relates to spark plugs for internal combustion engines, and to a method of making such spark plugs. More particularly, the present invention relates to a spark plug having a bimetallic ground electrode, and to a method of making such a spark plug.
2. Description of the Background Art
Spark plugs are widely used to ignite fuel in internal combustion engines. Spark plug electrodes are subject to intense heat, and to a highly corrosive environment, generated by the exploding air/fuel mixture. To improve durability and erosion resistance, spark plug electrodes must be able to withstand the high temperature and corrosive environment resulting from the chemical reaction products between air, fuel, fuel additives, and recirculated exhaust gases within a combustion chamber.
Spark plug designs have been suggested in which a bimetallic ground electrode includes a central core material, usually including copper, and a surrounding cladding material which is different from the central core material. This central core material is more thermally conductive than the surrounding cladding, and therefore conducts heat away from the firing tip of the ground electrode better than the previous designs. A cooler ground electrode is preferable because it does not erode as quickly as a hotter electrode. As a result, a cooler electrode contributes to a longer useful spark plug life.
Illustrative examples of this type of spark plug design may be found in U.S. Pat. No. 4,970,426, 5,210,457, 5,395,273, 5,530,313, 5,551,902, 5,675,209 and 5,866,973.
Although this type of design for bimetallic ground electrodes helps to ensure both thermal and electrical conductivity therethrough, a problem exists with this type of design, because the material making up the central core does not usually bond well, in a welding process, to the main spark plug shell, which is normally made of a ferrous material such as steel. The outer cladding material tends to weld to the steel shell better than the central core material.
Some efforts have been put forward to try and improve the weld between the ground electrode and the shell. U.S. Pat. No. 5,530,313 to Chiu discloses a method of welding a copper cored ground electrode to a metal spark plug shell, in which a metal sheath of the ground electrode surrounds a copper core, and in which the metal sheath penetrates deeper into the metal shell than the copper core to provide an anchor therefor. An electrode 116 which has a metal sheath 119 extending beyond a central copper core 117 is shown in FIG. 3 of the present specification. In attaching a prior art ground electrode, such as the electrode 116 shown in FIG. 3, to a flat surface of a shell base 115, the area of contact between the electrode sheath 119 and the shell base is still somewhat limited.
A need still exists in the art for an improved design for a spark plug having a bimetallic ground electrode, in which the welded connection between the ground electrode and the shell is further enhanced and improved.